@User The ones I am familiar with are FastCap and FastHenry which you probably know are open-source and pretty well documented. There are some details about how to use them specifically for inductors, but I don't recall the need for any particularly clever tricks (and if it was that clever then I probably ought not to disclose it!). I did also have to do a couple of very minor patches to them, which I can dig out. Basically we just had a script that made each segment of each turn be its own inductor element in FastHenry and its own conductor in FastCap, and then calculated every self- and mutual- inductance and capacitance, and made a big mess of a subcircuit, that the circuit simulator seemed pretty happy with most of the time. At the time I was using FastCap and FastHenry, FastFieldSolvers was just starting to provide a (closed source) 3D viewer (fastmodel) for the geomentry model files and that was very useful. I am not sure what the present state of that software is but from a quick look they seem to have made progress, and it seems that they have open-sourced their stuff which is really great. I recall that FastCap and FastHenry used to take overnight to do one inductor about 15 years ago, and that they were single-threaded. Perhaps someone (fastfieldsolvers.com ?) has made them multithreaded in the mean time, if not that could be worthwhile.